1. Field of the Invention
The present invention relates to a manufacture method of a photovoltaic element. More particularly, the present invention relates to a manufacture method of a photovoltaic element which can reduce photo-degradation attributable to interfacial levels, prevent peel-off or delamination between semiconductor layers, and provide high resistance against a reverse bias.
2. Description of the Related Art
Defect levels present in semiconductor layers making up a photovoltaic element are closely related to generation and recombination of electric charges and are one of factors affecting characteristics of the photovoltaic element.
As one example of methods for compensating such defect levels, hydrogen plasma treatment for photovoltaic elements has been proposed as follows.
(1) U.S. Pat. No. 4,113,514 (J. 1. Pankove et. al., RCA, Sep. 12, 1978) discloses hydrogen plasma treatment for completed semiconductor elements.
(2) xe2x80x9cEFFECT OF PLASMA TREATMENT OF THE TCO ON a-Si SOLAR CELL PERFORMANCExe2x80x9d, F. Demichelis et. al., Mat. Res. Soc. Symp. Proc., Vol. 258, p 905, 1992 describes a method of carrying out hydrogen plasma treatment on a transparent electrode deposited over a substrate and then forming a solar cell of pin-structure over the treated electrode.
(3) HYDROGEN-PLASMA REACTION FLUSHING FOR a-Si:H P-I-N SOLAR CELL FABRICATIONxe2x80x9d, Y. S. Tsuo et. al., Mat. Res. Soc. Symp. Proc., Vol. 149, p 471, 1989 describes that, in fabricating solar cells of pin-structure, hydrogen plasma treatment is performed on the surface of a p-layer before an i-layer is deposited.
In those proposals, the hydrogen plasma treatment is carried out by introducing only hydrogen gas to a treatment chamber and activating the hydrogen gas with radio frequency (hereinafter abbreviated to xe2x80x9cRFxe2x80x9d) to create a hydrogen plasma.
However, the hydrogen plasma not only acts on a substrate, an uncompleted element and a completed element which are to be treated, but also spreads over an entire inner wall of the chamber. Because the hydrogen plasma is very active, materials adsorbed on or contained in the inner wall surface of the chamber are sputtered therefrom by the hydrogen plasma. This raises a problem that if the sputtered materials are impurities (e.g., oxygen, nitrogen, carbon, iron, chromium, nickel and aluminum) which would give rise to defect levels when taken into semiconductor layers, generation and recombination of electric charges are disturbed and a photoelectric conversion characteristic is deteriorated.
Another problem is that because hydrogen gas is more difficult to discharge than other gases and a plasma created by hydrogen gas alone is more difficult to sustain than that created by using other gases, stable hydrogen plasma treatment cannot be ensured.
An object of the present invention is to provide a method for manufacturing a photovoltaic element with which, when carrying out, e.g., plasma treatment in manufacture of photovoltaic elements, impurities adsorbed on or contained in the inner wall surface of a treatment chamber will not be taken into the interior of and the interfaces between semiconductor layers, etc., and semiconductor layers having superior characteristics can be produced.
According to the present invention, in a method for manufacturing a photovoltaic element wherein a pin-structure formed by laminating n-, i- and p-type semiconductor layers, each of which contains silicon atoms and comprises a non-monocrystalline crystal structure, is formed at least one or more times on a substrate, the method comprises steps of forming each of the semiconductor layers and annealing the surface of at least one of the semiconductor layers or the substrate in an atmosphere of hydrogen gas, helium gas or argon gas that contains 1 to 1000 ppm of oxygen atom containing gas.